Let a session's transport say what it negotiated

ISshConnection and ISftpSession both carry Cipher now — the server-to-client
algorithm off SSH.NET's own ConnectionInfo, captured once because a rekey is
not an event that library raises — and TerminalWorkspace.GetSessionFacts hands
that plus the host key's algorithm back per session, without ever handing over
the connection itself. Nothing reads either yet; the status bar that will is
the next commit.

tests/DodoSSH.Client.Terminal.Tests/FakeShellSession.cs

@@ -136,6 +136,9 @@ internal sealed class FakeConnection(SshConnectionRequest request, long bytesPer
     public HostKeyPresentation HostKey { get; } =
         new(request.Host, request.Port, "ssh-ed25519", "SHA256:fake");
 
+    /// <inheritdoc />
+    public string Cipher { get; } = "aes256-gcm@openssh.com";
+
     /// <summary>The shell this connection opened, if it opened one.</summary>
     internal FakeShellSession? Shell { get; private set; }